In an optoelectronic transmission module with a laser diode there is the requirement for the latter to be electrically connected to a laser driver device. In this respect, it must be noted that, for transmission of high data rates between 10 and 40 Gbits/s, the radio-frequency line between the laser driver device and the laser diode must either be as short as possible or impedance-matched. In the case of an impedance-matched line, it must be endeavored here to minimize the number of connections, which in each case act as discontinuities and consequently as an “impedance jump” of the signal transmission.
A laser driver device is generally an integrated circuit (IC) which is designed for maximum speed by using bipolar technology and takes up a high power loss. Since a laser diode generally reacts very sensitively to higher temperatures, it must also be endeavored to provide thermal decoupling between the laser diode and the laser driver device. Furthermore, it must be endeavored to ensure that the position of the optical axis of the transmission component, and consequently the possibilities for the coupling of an optical fiber, are not restricted by mechanical boundary conditions arising as a result of the electrical contacting of the transmission module.
It is known for this purpose to arrange the laser driver device and the laser diode in a common package. Such packages are known by the designations butterfly package and mini-DIL package. The laser driver device can in this case be placed very close to the laser diode. To avoid strong thermal coupling between the driver device and the laser diode, the additional integration of a Peltier cooler, which cools the laser diode, is generally required. However, in uncooled operation, arrangements of this type have strong thermal coupling. They are also relatively cost-intensive to produce on account of high component costs. Furthermore, required hermetic fiber feed-throughs can only be realized with great effort and the pigtail fiber outputs that are usually used are disadvantageous because of the requirement to maintain minimum bending radii in the construction of compact optoelectronic modules of a small type.
For the electrical connection of a laser driver device to a laser diode, it is also known to arrange the laser driver device on a separate printed circuit board. The laser diode is arranged in a transistor outline (TO) package, to which an optical connector with an optical fiber can be coupled in the manner of a plug by means of a connector receptacle. The electrical connection between the TO package and the laser driver device arranged on the printed circuit board takes place by means of a flexible conductor.
Flexible conductors are arrangements known in the prior art in which conductor tracks are applied on one side, both sides or a number of sides on a planar flexible dielectric.
A corresponding arrangement is schematically represented in FIG. 4. FIG. 4 shows as the main components a TO package 400 with a laser diode, a flexible conductor 300 and a printed circuit board 500, on which a packaged driver device 200 is arranged as a surface-mounted device. The signal outputs of the driver device 200 to the laser diode are connected to impedance-matched conductor tracks 510 of the printed circuit board. By means of a soldered connection 520, the conductor tracks 510 of the printed circuit board 500 are connected to continuing conductor tracks 310 of the flexible conductor 300, which are likewise impedance-matched. The electrical connection between the flexible conductor 300 and the TO package 400 takes place by means of terminal pins 420 of the TO package 400, which protrude from a base plate 410 of the TO package 400 and are inserted through via holes 430 of the flexible conductor. In this respect, only one such terminal pin 420 is represented in FIG. 4 for better overall clarity.
The known arrangement provides good thermal decoupling between the laser device and the laser driver device. Use of a flexible conductor also has the effect that the TO package and the printed circuit board are mechanically decoupled, so that there is extensive freedom of design with regard to the alignment of the TO package, and consequently of the optical axis of the light emission with respect to the printed circuit board.
Furthermore, flexible conductors have the advantage that they have favorable RF properties at high frequencies in the GHz range. This is particularly important for transmission with a high data rate of 10–40 Gbits/s. For this case of a high rate, the RF lines must be impedance-matched. In this case, the conductor tracks of a flexible conductor in the coplanar design or in the microstrip design can be monitored in terms of their impedance and can therefore be used advantageously. The conductor tracks of the printed circuit board also have a monitorable impedance.
On the other hand, the large number of connecting points in the path of the signal is problematical. The connections between the TO package and the flexible conductor, between the flexible conductor and the printed circuit board and between the printed circuit board and the electrical component cannot be monitored in terms of their impedance and represent unavoidable discontinuities in the path of the signal. Particularly the soldered connection 520 between the flexible conductor 300 and the printed circuit board 500 makes an RF connection with data rates higher than 10 Gbits/s impossible.
A further disadvantage of the known arrangement is that the soldered connection 520 must satisfy conflicting requirements. Firstly, the solder pads of the soldered connection 520 are responsible for a mechanical connection between the flexible conductor 300 and the printed circuit board 500. To this extent, it must be endeavored to form the solder pads with the largest possible surface area. At the same time, the high-frequency signal connection to the TO package 400 takes place by way of the solder pads of the soldered connection 520. Accordingly, it must be endeavored to make the solder pads have the smallest possible surface area. In practice, relatively large solder pads are used, on the one hand to provide adequate mechanical stability and on the other hand to allow tolerances between the position of the solder pads of the printed circuit board and the position of the assigned solder pads of the flexible conductor to be taken into account. This leads to the soldered connection 520 representing a great discontinuity in the path of the signal.